A number of N-nitrosamines are powerful respiratory tract carcinogens which require metabolic activation in the host organism. This metabolic activation is believed to be mediated by cytochrome P-450 enzymes although unequivocal evidence for this hypothesis has not yet been achieved. An experiment was conducted to investigage the effect of the P-450 enzyme inhibitor, piperonyl butoxide, on the induction of lung tumors which originate from Clara cells in hamsters treated with N-nitrosodiethylamine (DEN). The effect of piperonyl butoxide on covalent binding and distribution of the parent nitrosamine was examined in vivo after 1 dose of C14-DEN. Moreover, the effect of piperonyl butoxide on the tumor incidence induced by DEN in a chronic study was investigaged by histopathology. Piperonyl butoxide significantly inhibited metabolism of DEN in the respiratory tract and inhibited the induction of lung tumors. These data provide the first experimental evidence for cytochrome P-450 enzymes being a crucial factor to the metabolic conversion of DEN into a carcinogenic metabolite in vivo. As a comparative approach, human lung cancer cell lines (HCCL0 with and without morphological features of pulmonary Clara cells were tested for their ability to metabolize DEN in vitro. It was found that the cell line with Clara cell features metabolized DEN to a significantly greater extent than cell lines derived from other types of lung cancers and that such metabolism was significantly inhibited by cytochrome P-450 inhibitors. Moreover, an HCCL with morphological features of alveolar type II cells was found to metabolize DEN to a great extent via prostaglandin endoperoxide synthetase. This is the first experimental evidence for different cell types in the lung to metabolize a nitrosamine via different pathways and may be potentially great importance in explaining the cell type-specific effects of nitrosamines.